Pluripotent embryonic stem (ES) cells are derived from the inner cell mass(ICM) and epiblast of mammalian embryos. Given their origin, do ES cell cultures contain cells at a single developmental stage or mixtures of cells at slightly different stages? On , Toyooka and colleagues report that these cultures actually contain subpopulations of cells that correspond to ICM, epiblast and primitive ectoderm (PrE). The ICM expresses Oct3/4 (a transcription factor that maintains pluripotency) and Rex1 (a marker of pluripotency); the PrE expresses only Oct3/4. By using ES cell lines in which genes for fluorescent proteins have been inserted into the Rex1 and Oct3/4 gene loci, the researchers identified subpopulations of Rex1+/Oct3/4+ (ICM-like) cells and Rex1-/Oct3/4+ (PrE-like) cells in undifferentiated ES cell cultures. These subpopulations can interconvert in vitro, they report, but have different differentiation potencies in vitro and in vivo. Given these results, the researchers suggest that their gene knock-in approach could help to identify the factor(s) that turn ICM into PrE.
